Rhodospirillum rubrum, an anoxygenic phototrophic bacterium, grows fermentatively in the dark and forms intracytoplasmic membranes, which resemble "chromatophores" in light-grown cells. Even though these dark-formed membranes contained bacteriochlorophyll a and appeared structurally normal, they did not exhibit photochemical reactions present in the membranes of photosynthetically-grown cells. As a result, when dark fermenting R. rubrum was first exposed to about 860 lux of white light, after dark growth, the cells exhibited a diauxic growth response. The additional observation, that diauxic growth could be virtually abolished during exposure to 6,500 lux of light, suggested that the inoperative, dark-formed membranes were activated during the first exponential growth response phase by certain amounts of radiant energy. To examine this hypothesis, the effect of radiant energy on the appearance of light-stimulated cytochrome c2 and 3, 3'-diaminobenzidine oxidation activities, and the development of photophosphorylation in the dark-formed membranes is being determined. It is believed that light-influenced cytochrome c2-association with the dark-formed membranes is a key event responsible for their functional differentiation. With this model, it may be possible to study functional differentiation (in light) and structural assembly (in darkness) of membranes as separable molecular events.